Date   

Re: TDS2000 brightness control

Mark Jordan
 

Or you can replace the backlight with white LEDs.

    Mark Jordan

On 01-Mar-20 19:49, Tom B wrote:
Unfortunately, it does not appear that there is any way to adjust the brightness.  They built the main power supply and CCFL inverter all on one board.  There is nothing to adjust.  The service manual specs the inverter output at 1400V P-P @5mA load.

I don't know if there is enough room inside to put a separate inverter for the backlight.  It might be better just to live with it the way it is.

Tom

On 3/1/2020 4:25 PM, Harvey White wrote:
You can do that, possibly.  It depends on whether or not the display has an easily replaceable CCFL bulb.

What you'd need would be an LED strip, the appropriate resistors and LEDs.  You'd need a variable PWM source or perhaps an additional resistor/variable voltage regulator to give you the brightness you want, or just adjust the resistors (which can get messy if you do it too many times).  You can adjust the resistors to work profitably on a 5 volt supply, for instance.

Another option is to replace the inverter with a variable brightness one, and put a small pot in to control that brightness.  If the existing configuration has the provisions for bright/dim the inverter, you may want to mess with that instead of putting LEDS in or replacing the inverter.

I have replaced CCFL lamps with LED strips, and do have LEDS and the blank PC boards, though.  You'll want to be able to do surface mount soldering, the parts are 1206 and 805 in size.

I'd look at messing with the control of the inverter first.  I would NOT mess with the high voltage part at all, you're talking 1600 volts at 8 ma or so to light that little CCFL lamp.

Likely the first thing would be to see what the inverter has in the way of a control voltage, for the ones I've seen, it's a simple voltage from, say, 0 to 4 volts.  Don't remember if more is less here, but that depends on the inverter.  Some are not brightness controlled, some are.


Re: Modifications to 577 Curve Tracer Main Power Supply

Ed Breya
 

OK, so with partial output to -7V, then it's likely the voltage on VR772 is stuck way too low (failed or dumped by C772), or Q772 is breaking down or leaking. If Q772 went open, it would go open loop overall, and the no-load output would be near the 40V raw supply. There are a number of ways it could have failed and makes -7V output, but I'd suspect the "772" parts at this point.

Yes of course Raymond, the output transistor would be turned on fully if it went open loop. I'm presuming it's still closed loop, with the failure limiting how high it can go. This appears to be the better kind of failure - not enough output rather than way too much, which could do some damage.

Ed


Re: TDS3000B series

David Kuhn
 

You know David, it is so sad, it's almost funny. One of the instruments
that I work on (non-oscope) used a backlight inverter similar, but
different, to the backlight converter used in the TDS3000 series o-scopes.
In the case of that inverter, it only drives one backlight tube and it is
not NEC. They have not made the converter for over 10 years and doesn't
make any now with the advent of LED backlighting. They are still around,
just doing other stuff. We asked them if we there is a schematic for the
backlight converter so we could possibly repair them at the
component level. They said "Yes". When I replied "Great, can you email it
to me?", they replied "Sorry, we do have a schematic, but it contains
industry secrets and we can not release the schematic."! They haven't
made the blasted thing in ten years and you can not find that one on ebay.
What would it hurt to release the schematics? I have worked around inside
those converters and managed to repair a couple, but it's not easy,
especially if it does work, just draws extra current and blows the
series +V coil occasionally.

Dave

On Sun, Mar 1, 2020 at 4:21 AM David C. Partridge <
david.partridge@perdrix.co.uk> wrote:

All that I know - I was hoping there might be something like a CLIP or
similar floating around.

D.





Re: CRT rebuilding

Jeff Kruth
 

Long ago, I did business with Thomas, a first class outfit. They needed glass for re-building a special tube they sold and I had a lot of it, so I traded twenty duds for two rebuilt tubes. They re-gun the tube as far as I know, cut off the old gun, re-phosphor and put in a new gun. Relabeled and everything. I dont know how they will do with custom CRTs from Tek. What if they break the CRT envelope? Maybe they would do it on a "If it works out, we will charge you" kind of effort. However, expect to pay something real. $300-400 dollars, maybe more, would not surprise me.

Jeff Kruth

In a message dated 3/1/2020 6:05:21 PM Eastern Standard Time, donfrasher=yahoo.com@groups.io writes:
 Looking over their web page FAQ they claim to do around 50 different phosphors.I also sent them a note from there contact listing. I'm thinking at this point that cost will be the big factor.

    On Sunday, March 1, 2020, 05:56:19 PM EST, John Williams <books4you@telus.net> wrote:

Interesting. I am going to contact them. If they can redo the phosphore maybe they could change it to a different one. Depending on cost of course. Thanks.


Re: Modifications to 577 Curve Tracer Main Power Supply

 

On Mon, Mar 2, 2020 at 12:08 AM, Ed Breya wrote:


Is the -30V zero, or some partial level?

Gary mentioned - 7 V.


There is no "lock-up" condition you alluded to - it should start up just fine,
unless there's a failure. If VR772 has no current from the -30V output via
R772, there should still be plenty (a few mA) to pull it down via R774, into a
regulation range where it will produce some output, and then get VR772's
operating current to the normal level. This arrangement may be to provide some
degree of foldback current limiting, but it's more likely for improvement of
line ripple rejection.
I gave my opinion on this a few posts ago. If I'm not mistaken, Q788 is in full conduction by R787 pulling to gnd, *unless throttled* by the regulating circuits.

Raymond


Re: Modifications to 577 Curve Tracer Main Power Supply

 

On Mon, Mar 2, 2020 at 12:04 AM, Raymond Domp Frank wrote:


... What about R776 or its connection to R775 being open..? I know you checked
everything but the mystery remains...
... and it would result in about - 7V (6.2 + 0.7) V at the output.

BTW: Sorry for the concatenation of short posts. I had intended and started to do something completely different but came back to this thread several times because it must be sooo simple that it's not only frustrating Gary.

Raymond


Re: Modifications to 577 Curve Tracer Main Power Supply

Ed Breya
 

Do you have enough access to make live measurements in this section while it's hooked up and powered? If so, you should be able to check some operating voltages and find the problem. Is the -30V zero, or some partial level?

If access during operation is very difficult, a good trick is to pick certain nodes, and tack probing wires onto them, long enough to be brought out into the open. You have to of course be very careful to not short any of these temporary test points.

There is no "lock-up" condition you alluded to - it should start up just fine, unless there's a failure. If VR772 has no current from the -30V output via R772, there should still be plenty (a few mA) to pull it down via R774, into a regulation range where it will produce some output, and then get VR772's operating current to the normal level. This arrangement may be to provide some degree of foldback current limiting, but it's more likely for improvement of line ripple rejection.

The usual suspects in old Tek gear of this vintage, are Ta caps. Only C781 and C772 appear to be candidates for close inspection. First determine what types they are, then act accordingly. If C772 is shorted (or partially shorted), for instance, then VR772's voltage would always be stuck near ground, and the -30V output would only reach a few volts. C781 would of course short the output, if failed short. The next suspect would be Q772 - if it's shorted or breaks down, the output will be cut off completely. And so on down the line.

You may have checked all these parts already, but not necessarily at operating conditions. That's why the best way to diagnose is with actual circuit measurements, directly or via temporary test points. I'd suggest the minimal set would be the bases of Q772 and Q786, which will tell a lot. If temporary TPs are needed, and it's difficult to remove and replace the PS board, then lots of wires will minimize the number of times this is needed. In this case, the BEC of Q772, the BC of Q786, and E of Q788 should be accessed. These should quite quickly and thoroughly show what's going on - it's actually a pretty simple circuit.

One last thing to consider is "mod failure" - if the various parts indicated with value changes or are tacked onto the circuit, look at them closely for proper installation and connections.

Good luck.


Re: CRT rebuilding

 

Looking over their web page FAQ they claim to do around 50 different phosphors.I also sent them a note from there contact listing. I'm thinking at this point that cost will be the big factor.

On Sunday, March 1, 2020, 05:56:19 PM EST, John Williams <books4you@telus.net> wrote:

Interesting. I am going to contact them. If they can redo the phosphore maybe they could change it to a different one. Depending on cost of course. Thanks.


Re: Modifications to 577 Curve Tracer Main Power Supply

 

On Sun, Mar 1, 2020 at 11:56 PM, Raymond Domp Frank wrote:


- 7 V is just about the voltage at the base of Q772 (and the slider of R775)
under normal circumstances...
... What about R776 or its connection to R775 being open..? I know you checked everything but the mystery remains...

It can't be a shorted R777, because you said that adjusting R775 does nothing. A shorted R775 would give almost - 9 V.

Raymond


Re: Modifications to 577 Curve Tracer Main Power Supply

 

On Sun, Mar 1, 2020 at 07:02 PM, Gary Robert Bosworth wrote:


This power supply does not start up correctly. The output is set at 7
Volts
- 7 V is just about the voltage at the base of Q772 (and the slider of R775) under normal circumstances...

Raymond


Re: CRT rebuilding

John Williams
 

Interesting. I am going to contact them. If they can redo the phosphore maybe they could change it to a different one. Depending on cost of course. Thanks.


CRT rebuilding

 

Just curious. I found a company that STILL does CRT rebuilding, wondering if anyone on the list has tried them or looked into it.

https://www.thomaselectronics.com/repair-overhaul/

Not sure if all they do is "rejuve" , or go as far as a full rebuild. Would be great for that aging P11

Don Frasher
AD7LL
donfrasher@yahoo.com
drfrasher@gmail.com


Re: TDS2000 brightness control

Tom B
 

Unfortunately, it does not appear that there is any way to adjust the brightness.  They built the main power supply and CCFL inverter all on one board.  There is nothing to adjust.  The service manual specs the inverter output at 1400V P-P @5mA load.

I don't know if there is enough room inside to put a separate inverter for the backlight.  It might be better just to live with it the way it is.

Tom

On 3/1/2020 4:25 PM, Harvey White wrote:
You can do that, possibly.  It depends on whether or not the display has an easily replaceable CCFL bulb.

What you'd need would be an LED strip, the appropriate resistors and LEDs.  You'd need a variable PWM source or perhaps an additional resistor/variable voltage regulator to give you the brightness you want, or just adjust the resistors (which can get messy if you do it too many times).  You can adjust the resistors to work profitably on a 5 volt supply, for instance.

Another option is to replace the inverter with a variable brightness one, and put a small pot in to control that brightness.  If the existing configuration has the provisions for bright/dim the inverter, you may want to mess with that instead of putting LEDS in or replacing the inverter.

I have replaced CCFL lamps with LED strips, and do have LEDS and the blank PC boards, though.  You'll want to be able to do surface mount soldering, the parts are 1206 and 805 in size.

I'd look at messing with the control of the inverter first.  I would NOT mess with the high voltage part at all, you're talking 1600 volts at 8 ma or so to light that little CCFL lamp.

Likely the first thing would be to see what the inverter has in the way of a control voltage, for the ones I've seen, it's a simple voltage from, say, 0 to 4 volts.  Don't remember if more is less here, but that depends on the inverter.  Some are not brightness controlled, some are.


Re: Modifications to 577 Curve Tracer Main Power Supply

 

...if the -30V does not exist, then the
reference diode is never turned on.
Addition: Strictly speaking, you *are* correct, of course. It's just that the - 30 V *does* exist without the regulation part being functional - or present. In fact, the voltage would probably try to go all the way to - 40 V. The regulation circuitry (mainly VR772, Q772 and Q786) *prevents* that by limiting to - 30 V.

Raymond


Re: Modifications to 577 Curve Tracer Main Power Supply

 

On Sun, Mar 1, 2020 at 11:04 PM, Gary Robert Bosworth wrote:


The reason why I do not like this design is because of the fact that the
-30V output is used to bias the 6.2V reference diode VR772. This poses a
classic lock-up condition in that if the -30V does not exist, then the
reference diode is never turned on.
I think that's incorrect:
Whenever - 40 V is present, Q788 is pulled into conduction by R787 (to gnd), causing the output voltage to move from 0 V towards - 30 V.
The regulating circuit *reduces* that conduction by increasing conduction of Q786.

Raymond


Re: Modifications to 577 Curve Tracer Main Power Supply

Gary Robert Bosworth
 

Dennis: The parts in faded print are R773 and C773. They are not on the
circuit board.
The circuit design is not well described in the service manual. No normal
voltages and currents are mentioned in the print.
The +40V and -40V unregulated are fine.
ALL of the components measured perfectly good including the rectifier
bridges and filter capacitors.
The power supply behaves the same whether the loading connectors are
attached or not.
All of the other supply voltages are good including +200V, +12V, -12V, and
+5V.
I momentarily disconnected the short-circuit feedback diode CR785 when the
loads were not attached and it had no effect.
The reason why I do not like this design is because of the fact that the
-30V output is used to bias the 6.2V reference diode VR772. This poses a
classic lock-up condition in that if the -30V does not exist, then the
reference diode is never turned on.
I have measured every circuit trace from beginning to end and there are no
micro-cracks to open them up, and there are no solder bridges causing short
circuits.
This is a perplexing problem that I have not seen in 56 years of working in
electronics.

Gary

On Sun, Mar 1, 2020 at 11:53 AM Dennis Tillman W7PF <dennis@ridesoft.com>
wrote:

Hi Gary,
Which parts on which schematics are shown in shaded text?

Dennis Tillman W7pF
TekScopes Moderator



--
Gary Robert Bosworth
grbosworth@gmail.com
Tel: 310-317-2247


Re: Repairing my Tektronix 454 oscilloscope need some advice (NOW ALIVE !!)

Albert Otten
 

Good news Victor, congratulations!
I think my 454 has no net filter at all, could a filter be present in later S/N?
Albert


Re: TDS2000 brightness control

Harvey White
 

You can do that, possibly.  It depends on whether or not the display has an easily replaceable CCFL bulb.

What you'd need would be an LED strip, the appropriate resistors and LEDs.  You'd need a variable PWM source or perhaps an additional resistor/variable voltage regulator to give you the brightness you want, or just adjust the resistors (which can get messy if you do it too many times).  You can adjust the resistors to work profitably on a 5 volt supply, for instance.

Another option is to replace the inverter with a variable brightness one, and put a small pot in to control that brightness.  If the existing configuration has the provisions for bright/dim the inverter, you may want to mess with that instead of putting LEDS in or replacing the inverter.

I have replaced CCFL lamps with LED strips, and do have LEDS and the blank PC boards, though.  You'll want to be able to do surface mount soldering, the parts are 1206 and 805 in size.

I'd look at messing with the control of the inverter first.  I would NOT mess with the high voltage part at all, you're talking 1600 volts at 8 ma or so to light that little CCFL lamp.

Likely the first thing would be to see what the inverter has in the way of a control voltage, for the ones I've seen, it's a simple voltage from, say, 0 to 4 volts.  Don't remember if more is less here, but that depends on the inverter.  Some are not brightness controlled, some are.

Harvey

On 3/1/2020 4:05 PM, Tom B wrote:

Yes, that is what I got.  They do work in the TDS2000 but I think the backlight is a bit too bright.  There doesn't seem to be any way to change it short of getting rid of the CFL bulb and putting in LEDs.

Tom


On 3/1/2020 3:49 PM, satbeginner wrote:
Using Google I found this:


https://urldefense.com/v3/__https://www.goldmine-elec-products.com/prodinfo.asp?number=G19815__;!!KfGNmQmE!2zITbJD8FLS3gsRdikglcjy87OGTVu2u5BokwWCSWXYZg5hobW2T6AVS4oDo$



Re: TDS2000 brightness control

satbeginner
 

Usually these CFL inverters used for the backlight (at least in laptops) have an analog pin to adjust the brightness of the CFL, also there should be some room to play with using the contrast setting of the LCD itself.
I do not have such a device, but I changed both display and inverter in a Tektronix THS710 a long time ago having to do something similar.

Succes,

Leo


Re: TDS2000 brightness control

Tom B
 

Yes, that is what I got.  They do work in the TDS2000 but I think the backlight is a bit too bright.  There doesn't seem to be any way to change it short of getting rid of the CFL bulb and putting in LEDs.

Tom

On 3/1/2020 3:49 PM, satbeginner wrote:
Using Google I found this:


https://urldefense.com/v3/__https://www.goldmine-elec-products.com/prodinfo.asp?number=G19815__;!!KfGNmQmE!2zITbJD8FLS3gsRdikglcjy87OGTVu2u5BokwWCSWXYZg5hobW2T6AVS4oDo$

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